Titanium-added, high strength steel

ABSTRACT

Disclosed is a titanium(Ti)-added steel wherein the formation of TiN or nitrogen-rich TiCN, which is found in a conventional titanium-added steel for machine construction and adversely affects the properties of the titanium-added steel for machine construction, has been suppressed, particularly a titanium-added, high strength steel wherein various properties can be stably exhibited and Ti(C)N has been regulated.  
     This titanium-added, high strength steel is a steel for machine construction, which comprises, as steel constituents, by weight, titanium: not less than 500 ppm, the content of nitrogen (N) being N&lt;100 ppm and has excellent fatigue limit as shown in FIG.  1.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a titanium(Ti)-added, high strength steel as a steel for machine construction for use in automobile components and components of other various industrial machines or apparatuses.

[0003] 2. Background Art

[0004] Steels for machine construction, for example, SC steel, SMn steel, SCr steel, SCM steel, SNC steel, SNCM steel, and SUJ steel specified in JIS (Japanese Industrial Standards) and the above steels with boron and/or minor elements being further added thereto have hitherto been used in automobile components and components of other various industrial machines or apparatuses. Regarding these steels for machine construction, steels, in which titanium in an amount of not less than 500 ppm has been added as a steel constituent, are known (see, for example, Japanese Patent Laid-Open Nos. 283910/1996, 130720/1998, 251806/1998, 293403/1999, and 293392/1999). These publications disclose that the addition of titanium as a steel constituent in an amount of not less than 500 ppm can realize steels which are excellent in various properties, for example, static strength, fatigue strength, and grain size characteristics. Even in these titanium-added steels for machine construction, however, in order to stably provide the above properties, problems associated with steel making should be solved. That is, the formation of TiN or nitrogen(N)-rich TiCN as an inclusion, which adversely affects the properties, should be suppressed, and, further, also in the production process, the crystallization of large size TiN, which is not found in general steels and is inherent in titanium-added steels, should be prevented.

SUMMARY OF THE INVENTION

[0005] It is an object of the present invention to provide a titanium-added steel for machine construction, that could have solved the problem of the formation of TiN or nitrogen-rich TiCN inclusions, which adversely affects steel properties and could not have been satisfactorily solved in the conventional titanium-added steels, that is, has suppressed the formation of TiN or nitrogen-rich TiCN inclusions, particularly a titanium-added, high strength steel in which the crystallization of Ti(C)N has been regulated for stably providing various properties.

[0006] In order to attain the above object, according to the present invention as defined in claim 1, there is provided a titanium-added, high strength steel as a steel for machine construction, comprising, by weight, titanium: not less than 500 ppm as a steel constituent, the content of nitrogen (N) being N<100 ppm.

[0007] According to the present invention as defined in claim 2, there is provided a titanium-added, high strength steel as a steel for machine construction, comprising, by weight, titanium: not less than 500 ppm, the maximum size of TiN and/or TiCN crystallized in the steel, {square root}{square root over (area max)}, being not more than 80 μm as predicted in a measurement area of 30000 mm² by an extreme value statistical method.

[0008] According to the present invention as defined in claim 3, there is provided a titanium-added, high strength steel as a steel for machine construction, comprising, by weight, titanium: not less than 500 ppm, the content of nitrogen (N) being N<100 ppm, the maximum size of TiN and/or TiCN crystallized in the steel, {square root}{square root over (area max)}, being not more than 80 μm as predicted in a measurement area of 30000 mm² by an extreme value statistical method.

[0009] Here {square root}{square root over (area max)} is the square root of the area of the maximum inclusion non-metallic inclusion present in a measurement area predicted by an extreme value statistical method, and, in the present invention, refers to the maximum size of TiN and/or TiCN which has been crystallized in the steel.

[0010] The steel as the basis in the means of the present invention is a steel for machine construction, and the object steel of the present invention is a steel for machine construction such that titanium has been added to the base steel for machine construction and the nitrogen content has been regulated.

[0011] The steel for machine construction as the base steel in the present invention is preferably based on a steel selected from the group consisting of SC steel (JIS G 4051 (1979)), SMn steel (JIS G 4106 (1979)), SCr steel (JIS G 4104 (1979)), SCM steel (JIS G 4105 (1979)), SNC steel (JIS G 4102 (1979)), SNCM steel (JIS G 4103 (1979)), and SUJ steel (JIS G 4805 (1999) ) and further comprises titanium and nitrogen and optionally minor elements with the balance comprising unavoidable impurities and satisfies the above requirements for titanium, nitrogen, and, in addition, TiC and TiCN.

[0012] The term “minor elements” used herein refers to elements which, when contained in an amount of not more than about 0.5%, offer an effect beneficial to the steel. Further, the term “optionally” means that the minor elements may be contained according to need depending upon applications, or may not be contained at all. Representative examples of minor elements will be described. Aluminum (Al) is an element which is frequently used as a deoxidizing element and may be contained in an amount up to 0.05%. Boron (B) is an element which improves harden ability and may be contained in an amount up to 50 ppm. Lead (Pb), bismuth (Bi), tellurium (Te), and selenium (Se) are elements which improve machinability and may be contained in an amount up to 0.3%. Likewise, calcium (Ca) may be contained in an amount up to 0.010%, and sulfur (S) may be contained in an amount up to 0.3%.

[0013] In these steels of the present invention, the nitrogen content is N<100 ppm by weight, preferably N<80 ppm by weight. In general, the titanium-added steel possesses excellent properties by virtue of TiC or carbon(C)-rich TiCN which has been finely precipitated in a size of not more than 100 nm by the addition of titanium. Titanium, however, reacts with nitrogen and carbon and consequently causes the crystallization of non-metallic inclusions of TiN or nitrogen-rich TiCN (TiN and nitrogen-rich TiCN being hereinafter collectively referred to as “TiN”). The crystallized non-metallic inclusions adversely affect the properties of the steel. Therefore, preferably, the crystallization of TiN is suppressed to bring the added titanium to precipitated TiC or carbon-rich TiCN having a size of not more than 100 nm which has a useful effect on the properties of the steel. To this end, in the present invention as defined in claim 1, the content of nitrogen contained in the steel is limited to N<100 ppm by weight, preferably N<80 ppm by weight.

[0014] Further, in the means as defined in claim 2 or 3 of the present invention, in particular, the maximum size ({square root}{square root over (area max)}) of TiN is regulated, and the maximum size ({square root}{square root over (area max)}) of TiN is brought to not more than 80 μm, preferably not more than 60 μm. There is a possibility that the crystallized TiN serves as the origin of fatigue fracture. Whether or not the crystallized TiN serves as the origin of fatigue fracture is determined by the relationship of the size of the crystallized TiN and the size of oxide inclusions contained in the steel. When the maximum size of TiN exceeds 80 μm, there is a high possibility that this size is larger than the size of the oxide inclusions. Therefore, TiN serves as the origin of fatigue fracture, and, under service conditions such that inclusion-originated fatigue fracture occurs, the strength of the titanium-added steel is lower than the strength of the steel to which titanium has not been added. When the maximum size of TiN is brought to not more than 80 μm, the size of TiN is equal to or smaller than the size of the oxide inclusions and, in this case, the above lowering in strength does not occur.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a graph showing the relationship between the nitrogen content and the fatigue limit of steels of heat No. described in Example 1 for steels of the present invention as defined in claim 1 and steels for comparison with the steels of the present invention;

[0016]FIG. 2 is a graph showing the relationship between the fatigue limit and the maximum size of TiN, {square root}{square root over (area max)}, in a rotary bending test of steels of heat described in Example 2 for steels of the present invention as defined in claims 2 and 3 and steels for comparison with the steels of the present invention; and

[0017]FIG. 3 is a graph showing the relationship between the probability of TiN as the origin of fracture and the maximum size of TiN, {square root}{square root over (area max)}, of steels of heat described in Example 2 for steels of the present invention as defined in claims 2 and 3 and steels for comparison with the steels of the present invention.

EMBODIMENTS OF THE INVENTION

[0018] Embodiments of the present invention will be described.

[0019] Steels having specified chemical compositions were melted in an electric furnace. The molten steels were then subjected to ladle refining. In this case, the amount of alloy in the molten steel was regulated, and about 0.015 to 0.023% of aluminum was added to the molten steel to deoxidize the molten steel with aluminum, thereby reducing the oxygen content. The molten steels were further subjected to RH degassing treatment. Titanium was added to the molten steels at the end of the degassing treatment. While the molten steel was circulated to a circulation amount twice the amount of the molten steel, the titanium content was brought to 500 to 2000 ppm. Thus, the molten steels (each 150 tons) having respective chemical compositions shown in Tables 1 to 3 were produced by the melt process.

[0020] These molten steels were poured at 1570° C. from the ladle into a tundish and were cast by a continuous casting apparatus into blooms having a large section of 380 mm×450 mm. In this case, blooms were drawn at a casting speed of 0.45 m/sec and were cut by a gas cutting apparatus at a position 36 m below the casting nozzle to prepare blooms. The blooms thus obtained were transferred to a billeting process. The time taken between the passage of the molten steel through the casting nozzle and the gas cutting was 80 min. These blooms were cut, and the cut surface was polished and was then corroded with an aqueous HCl solution. The metal structure of the corroded surface was observed, and the cooling rate was estimated from spacings of the dendrite arms. As a result, it was found that the cooling rate was about 1.1° C./min even at a portion around the center of the bloom which was solidified lastly.

[0021] In the billeting process, the blooms were heated to a temperature of 1180° C. or above and were then hot rolled to billets of 150 mmφ. In subsequently rolling the steel products, the billets were heated to a temperature of 1180° C. or above, and, after the rolling, the rolled steels are rapidly cooled to 1100° C. or below to prevent precipitates of TiC or carbon-rich TiCN from being grown to larger diameter grains. When the rolling time is short and, at the same time, the dimension of the steel after the rolling is small, the steel may be air cooled after the rolling. Otherwise, the rolled steel may be rapidly cooled, for example, by water cooling or air-blast cooling to suppress the growth of grains. When the as-rolled steel is not re-heated until it is brought to the final component, the above control suffices for contemplated quality. On the other hand, when re-heating is carried out after rolling for performing, for example, hot forging into components, as with the above-described hot rolling, the temperature control for suppressing grain growth is necessary.

EXAMPLES Example 1

[0022] In producing steel products of titanium-added steels as described in the embodiments of the present invention, in this example, JIS (Japanese Industrial Standards) SCr 420-based, SCM 420-based, SNCM 420-based, SNC 415-based, S45C-based, SMn 443-based, and SUJ 2-based titanium-added steels, and SCr 420-based boron-and-titanium-added steels were produced by a melt process. In this case, degassing time was varied so that the produced steels were different from one another in nitrogen content. The resultant blooms were heated and stretched to 20 mmφ, followed by normalizing. The normalized products were machined to prepare test pieces for a rotary bending test. In order to harden the surfaces of test pieces, quenching and tempering were performed for the SUJ 2-based titanium-added steel, high frequency quenching and tempering were performed for S45C-based titanium-added steel and SMn 443-based titanium-added steel, and carburization quenching and tempering were performed for the other titanium-added steels. For the test pieces, the surface of the test part was then subjected to abrasive finishing, and the test pieces were then tested with an Ono rotary bending fatigue testing machine. The chemical composition of each sample is shown in Table 1, and test results are shown in FIG. 1. For each heat No. in Table 1, the base steel and chemical composition thereof will be described. For heat No. 1, the base steel is SCr 420, and nickel (Ni) and molybdenum (Mo) are unavoidable impurities. For heat No. 2, the base steel is SCM 420, and nickel is an unavoidable impurity. For heat No. 3, the base steel is SNCM 420. For heat No. 4, the base steel is SNC 415, and molybdenum is an unavoidable impurity. For heat No. 5, the base steel is S45C, and nickel, chromium (Cr), and molybdenum are unavoidable impurities. For heat No. 6, the base steel is SMn 443, and nickel, chromium, and molybdenum are unavoidable impurities. For heat No. 7, the base steel is SUJ 2, and nickel and molybdenum are unavoidable impurities. For heat No. 8, the base steel is SUJ 2, boron is an additive element, and nickel and molybdenum are unavoidable impurities. Regarding the relationship between the nitrogen content and the rotary bending fatigue strength, it is apparent from FIG. 1 that, when the nitrogen content exceeds 80 ppm, the fatigue limit (10⁷ cycle fatigue strength) begins to lower; and, when the nitrogen content is not less than 100 ppm, the fatigue limit greatly lowers. This tendency is considered attributable to an increase in TiN harmful to the fatigue strength with increasing the nitrogen content. TABLE 1 Unit: weight %; Ti, O, N, and B in ppm Heat No. C Si Mn P S Ni Cr Mo Ti O N B Remarks 1 1 0.20 0.25 0.80 0.014 0.018 0.06 1.13 0.02 1459 9 90 — Claim 1 2 0.21 0.25 0.81 0.015 0.018 0.06 1.13 0.02 980 8 72 — Claim 1 3 0.20 0.25 0.80 0.014 0.017 0.05 1.14 0.02 580 9 66 — Claim 1 4 0.21 0.26 0.80 0.013 0.018 0.06 1.13 0.02 1525 9 102 — Comp. steel 5 0.21 0.25 0.80 0.014 0.018 0.06 1.13 0.02 1501 10 110 — Comp. steel 6 0.20 0.26 0.79 0.013 0.018 0.05 1.14 0.02 1133 10 81 — Claim 1 7 0.20 0.26 0.79 0.014 0.017 0.06 1.13 0.02 1942 10 77 — Claim 1 2 1 0.20 0.25 0.82 0.013 0.017 0.07 1.11 0.15 1378 8 61 — Claim 1 2 0.21 0.25 0.79 0.012 0.019 0.07 1.15 0.15 1482 9 115 — Comp. steel 3 1 0.21 0.23 0.55 0.015 0.016 1.61 0.51 0.15 1466 9 65 — Claim 1 2 0.20 0.25 0.55 0.015 0.018 1.61 0.52 0.15 1527 10 109 — Comp. steel 4 1 0.17 0.23 0.51 0.015 0.017 1.97 0.40 0.01 1472 10 67 — Claim 1 2 0.17 0.25 0.51 0.017 0.018 1.99 0.40 0.01 1390 9 115 — Comp. steel 5 1 0.45 0.27 0.82 0.016 0.018 0.08 0.12 0.01 1499 8 66 — Claim 1 2 0.45 0.26 0.82 0.014 0.018 0.08 0.12 0.01 1523 8 107 — Comp. steel 6 1 0.42 0.23 1.55 0.013 0.015 0.09 0.13 0.01 1377 9 71 — Claim 1 2 0.41 0.24 1.53 0.012 0.016 0.09 0.14 0.01 1456 9 109 — Comp. steel 7 1 1.00 0.24 0.44 0.010 0.008 0.10 1.42 0.01 1271 7 61 — Claim 1 2 1.00 0.23 0.45 0.010 0.008 0.10 1.41 0.01 1362 7 111 — Comp. steel 8 1 0.20 0.25 0.75 0.014 0.013 0.05 1.14 0.01 1451 9 66 13 Claim 1 2 0.21 0.26 0.76 0.013 0.015 0.06 1.13 0.02 1508 9 120 16 Comp. steel

Example 2

[0023] In producing steel products of titanium-added steels as described in the embodiments of the present invention, in this example, titanium-added steels based on steels specified in JIS were produced by continuous casting. In this case, solidification speed was controlled to vary the size of TiN to produce titanium-added steels with varied TiN size. Samples were then prepared from the produced steel products. The chemical compositions of the titanium-added steels are shown in Table 2. For each heat in Table 2, the base steel and chemical composition thereof will be described. For heat A, the base steel is SCr 420, and nickel and molybdenum are unavoidable impurities. For heat B, the base steel is SCM 420, and nickel is an unavoidable impurity. For heat C, the base steel is SNCM 420. For heat D, the base steel is SNC 415, and molybdenum is an unavoidable impurity. For heat E, the base steel is S45C, and nickel, chromium, and molybdenum are unavoidable impurities. For heat F, the base steel is SMn 443, and nickel, chromium, and molybdenum are unavoidable impurities. For heat G, the base steel is SUJ 2, and nickel and molybdenum are unavoidable impurities. For heat H, the base steel is SUJ 2, boron is an additive element, and nickel and molybdenum are unavoidable impurities. TABLE 2 Unit: weight %; Ti, O, N, and B in ppm Heat C Si Mn P S Ni Cr Mo Ti O N B A 1 0.20 0.26 0.80 0.015 0.018 0.06 1.13 0.02 1556 10 64 — 2 0.20 0.26 0.79 0.014 0.018 0.06 1.13 0.02 920 9 87 — 3 0.20 0.25 0.80 0.015 0.017 0.06 1.13 0.02 1446 12 71 — 4 0.21 0.25 0.79 0.15 0.018 0.06 1.14 0.02 1440 11 102 — 5 0.20 0.25 0.79 0.014 0.017 0.06 1.12 0.03 1901 10 128 — 6 0.20 0.25 0.80 0.014 0.018 0.06 1.13 0.02 568 10 70 — 7 0.20 0.25 0.81 0.014 0.017 0.06 1.13 0.02 1295 11 62 — 8 0.21 0.25 0.79 0.014 0.017 0.07 1.12 0.02 1498 10 112 — 9 0.21 0.25 0.80 0.015 0.017 0.06 1.13 0.02 1522 12 74 — 10 0.20 0.25 0.79 0.015 0.018 0.06 1.13 0.02 1484 10 63 — 11 0.20 0.25 0.80 0.015 0.017 0.05 1.13 0.02 1154 11 84 — 12 0.20 0.25 0.80 0.015 0.018 0.06 1.13 0.02 1446 10 114 — B 1 0.20 0.25 0.81 0.013 0.017 0.07 1.12 0.16 1654 8 77 — 2 0.21 0.26 0.79 0.011 0.016 0.09 1.16 0.15 1427 8 111 — C 1 0.22 0.25 0.54 0.017 0.016 1.62 0.52 0.15 1167 9 108 — 2 0.20 0.25 0.56 0.015 0.018 1.63 0.51 0.15 1361 10 99 — D 1 0.17 0.25 0.57 0.016 0.017 1.99 0.42 0.01 1098 11 88 — 2 0.17 0.25 0.55 0.016 0.018 1.97 0.44 0.01 1762 8 62 — E 1 0.45 0.27 0.85 0.015 0.016 0.06 0.11 0.01 1468 8 70 — 2 0.45 0.27 0.81 0.014 0.017 0.08 0.12 0.01 1490 8 120 — F 1 0.41 0.23 1.58 0.011 0.017 0.09 0.16 0.01 1478 9 65 — 2 0.42 0.25 1.55 0.016 0.018 0.10 0.15 0.01 1522 9 101 — G 1 1.00 0.25 0.40 0.010 0.008 0.10 1.40 0.01 1548 8 105 — 2 1.00 0.23 0.44 0.011 0.007 0.10 1.42 0.01 1422 6 67 — H 1 0.20 0.26 0.77 0.017 0.011 0.05 1.17 0.01 1444 9 67 15 2 0.21 0.27 0.77 0.017 0.015 0.06 1.16 0.02 1633 9 108 18

[0024] Specifically, samples were prepared from the steel products of titanium-added steels by heating and stretching the steel products to 20 mmφ, normalizing the stretched products, and then machining the normalized products to prepare test pieces for a rotary bending test which were then used for the test. The results of the rotary bending test are shown in FIG. 2. In FIG. 2, represents data for medium carbon steel. It is apparent from FIG. 2 that, when {square root}{square root over (area max)} exceeds 60 μm, the fatigue limit lowers; and, when {square root}{square root over (area max)} exceeds 80 μm, the fatigue limit further lowers.

[0025] For all the test pieces of the steel products shown in Table 2, non-metallic inclusions as a fracture origin were investigated, and {square root}{square root over (area max)}, the fatigue limit, and the probability of TiN as the fracture origin are shown in Table 3. Further, in FIG. 3, the probability of TiN as the fracture origin is plotted as the ordinate, and {square root}{square root over (area max)} of TiN is plotted as the abscissa. It is apparent from FIG. 3 that, as {square root}{square root over (area max)} of TiN increases, the probability of TiN as the fracture origin increases; and, when {square root}{square root over (area max)} of TiN exceeds 80 μm, in most cases, the fracture occurs with TiN serving as the fracture origin.

[0026] Specifically, when {square root}{square root over (area max)} is not more than 60 μm, the size of TiN is smaller than the size of oxide inclusions. Therefore, in this case, the oxide inclusions serve as the fracture origin, and the titanium-based inclusions do not affect the fatigue strength. On the other hand, when {square root}{square root over (area max)} is not less than 80 μm, the size of TiN is larger than the size of oxide inclusions. Therefore, in this case, in most cases of the fatigue fracture, TiN serves as the fracture origin, resulting in lowered strength. This demonstrates that the size of TiN affects the fracture strength. TABLE 3 Probability Heat

Fatigue limit of TiN origin Remarks A 1 36.5 1145 0 Claim 3 2 41.2 1148 0 Claim 3 3 44.1 1140 5 Claim 3 4 49.2 1141 5 Claim 2 5 59.0 1144 10 Claim 2 6 67.0 1122 40 Claim 3 7 69.2 1115 35 Claim 3 8 74.1 1118 60 Claim 2 9 75.0 1110 75 Claim 3 10 81.9 1095 99.5 Comp. steel 11 88.6 1097 100 Comp. steel 12 97.8 1090 100 Comp. steel B 1 57.3 1161 15 Claim 3 2 73.3 1120 70 Claim 2 C 1 68.0 1125 50 Claim 3 2 102.0 1080 100 Comp. steel D 1 66.0 1131 60 Claim 3 2 72.0 1130 80 Claim 3 E 1 56.3 652 10 Claim 3 2 88.8 606 100 Claim 2 F 1 57.9 631 25 Claim 3 2 80.4 613 100 Comp. steel G 1 71.0 1115 75 Claim 2 2 92.5 1087 100 Comp. steel H 1 76.7 1115 85 Claim 3 2 65.9 1133 55 Claim 2

[0027] As is apparent from the foregoing description, according to the present invention, a high strength steel having improved fatigue strength for machine construction can be realized by a steel wherein titanium: not less than 500 ppm is contained as a steel constituent, and the nitrogen content is N<100 ppm; a steel where in titanium: not less than 500 ppm is contained as a steel constituent, and the maximum size of TiN and/or TiCN crystallized in the steel, {square root}{square root over (area max)}, is not more than 80 μm as predicted in a measurement area of 30000 mm² by an extreme value statistical method; and a steel simultaneously satisfying the requirements for both the above steels, that is, a steel wherein titanium: not less than 500 ppm is contained as a steel constituent, the nitrogen content is N<100 ppm, and the maximum size of TiN and/or TiCN crystallized in the steel, {square root}{square root over (area max)}, is not more than 80 μm as predicted in a measurement area of 30000 mm2 by an extreme value statistical method. 

What is claimed is:
 1. A titanium(Ti)-added, high strength steel as a steel for machine construction, comprising, by weight, titanium: not less than 500 ppm, the content of nitrogen (N) being N<100 ppm.
 2. A titanium(Ti)-added, high strength steel as a steel for machine construction, comprising, by weight, titanium: not less than 500 ppm, the maximum size of TiN and/or TiCN crystallized in the steel, {square root}{square root over (area max)}, being not more than 80 μm as predicted in a measurement area of 30000 mm² by an extreme value statistical method.
 3. A titanium(Ti)-added, high strength steel as a steel for machine construction, comprising, by weight, titanium: not less than 500 ppm, the content of nitrogen (N) being N<100 ppm, the maximum size of TiN and/or TiCN crystallized in the steel, {square root}{square root over (area max)}, being not more than 80 μm as predicted in a measurement area of 30000 mm² by an extreme value statistical method.
 4. The titanium-added, high strength steel according to any one of claims 1 to 3, which is a steel based on a steel selected from the group consisting of SC steel (JIS (Japanese Industrial Standards) G 4051 (1979)), SMn steel (JIS G 4106 (1979)), SCr steel (JIS G 4104 (1979)), SCM steel (JIS G 4105 (1979)), SNC steel (JIS G 4102 (1979)), SNCM steel (JIS G 4103 (1979)), and SUJ steel (JIS G 4805 (1999)) and further comprising titanium and nitrogen and optionally minor elements with the balance comprising unavoidable impurities. 